1. Field of the Invention
The present invention relates to an isolation amplifier utilizing transformer coupling.
2. Description of the Prior Art
In various fields such as industrial process control systems, medical measurements, and the like, isolation amplifiers are used for coupling sensors to signal processors. In one such type of isolation amplifier, a modulated carrier signal is supplied to the primary winding of an isolation transformer. A secondary winding output signal is demodulated in synchronism with the input carrier to detect the analog input.
A double-balanced modulation method is known wherein positive and negative amplitude modulated pulses are supplied to the primary winding transformer in response to transmission signals. This method has the advantage of not requiring a rectification smoothing circuit or a low-pass filter (carrier suppressor), resulting in very wide band transmission characteristics. The pulse modulator may be of the switching type driven by the carrier pulse. Bipolar or field effect transistors (FET) are most commonly employed as the switching elements in integrated devices.
When transistors are used as switching elements of the modulator as described above, their ON resistance of several hundred ohms, combines with the inductance of the isolation transformer's primary winding to create an LR time constant which results in a time delay of the leading and trailing edges of the transmitted signal. When this signal is rectified, the resulting signal is discontinuous. Carrier noise (or carrier leakage) results at each gap between the leading and trailing edges of the carrier pulses. In order to control such carrier leakage, a low-pass filter must be connected to the output of the demodulator. However, this results in low frequency response, narrow transmission range, and poor linearity.
Accordingly, it is an object of the present invention to provide an isolation amplifier wherein carrier leakage in the output can be prevented without the need of a low-pass filter.
Another object is to provide such an isolation amplifier which has wide range transmission characteristics, low distortion, and high precision.
Another object is to provide such an amplifier which efficiently removes carrier leakage.
Other and further objects, features and advantages of the invention will appear more fully from the following description.